Latest news from the LHC

Over the past two weeks the LHC operations team has focused on pushing the LHC’s performance into new territory in terms of stored beam power. Moving to 25 bunches per beam with almost nominal bunch intensities at the end of July implied operation with a stored energy in each beam of more than 1 MJ. This corresponds to the current record for stored beam energy in existing hadron accelerators (e.g. CERN’s SPS and Fermilab’s Tevatron) and it marks an energy regime where a sudden loss of beam or operational errors can result in serious damage to equipment: an energy of 1 MJ is sufficient to melt 2 kg of copper. Extreme care and a thorough optimization of all operational procedures are therefore required in making this important transition in the machine’s performance.

The focus of the past two weeks has been on optimizing the operational procedures and the machine protection systems, with the aim of gaining experience with the reliability and reproducibility of the operation of the machine at such a high stored beam energy. This effort also featured record results for the LHC performance in terms of delivered luminosity. For the first time the peak luminosity surpassed 4x1030 cm-2 s-1 and the total integrated luminosity delivered to the experiments passed the milestone of 1 inverse picobarn (1 pb-1 or 1000 nb-1) over the weekend 7-8 August.

In parallel the operations team conducted several tests for improving the LHC performance still further. The ramp speed of the magnets (the rate at which the electrical current can be changed in the LHC main dipoles) has been increased from 2 A/s to 10 A/s for the pre-cycle (without beam) of the magnet system. The ramp speed of 10 A/s has also been successfully tested for acceleration with beam, but the final implementation must wait until the LHC starts operation with bunch “trains”, in which the bunches of protons are grouped closely together, in contrast to the present operation with widely separated bunches. The faster ramp speed reduces significantly the minimum required time between two physics fills and therefore increases the overall machine performance in terms of integrated luminosity.

Another step towards higher luminosity occurred just recently, when the number of bunches in each beam was increased from 25 to 49 on Thursday 19 August. This will be followed by a change to operation with bunch trains in September. Operating the machine with bunch trains will open the door for increasing the total number of bunches in successive steps, so improving the LHC’s luminosity over the coming months by another factor of 10 to 100.